CH 25 The Digestive System

Regions of the Small Intestine

The small intestine has three anatomical divisions: the duodenum, jejunum, and ileum.

Enteroendocrine Cells and G Cells

*Enteroendocrine cells* are quite numerous in the fundus of the stomach, but are only occasionally found within the cardia and pyloric part. In the fundus they are scattered among the parietal and chief cells. *G cells* are enteroendocrine cells found in the gastric pits of the pyloric part. They secrete the hormone *gastrin*. Gastrin, which is released when food enters the stomach, stimulates the secretory activity of both parietal and chief cells. It also promotes smooth muscle activity in the stomach wall that enhances mixing and churning activity.

Mucous Neck Cells

*Mucous neck cells* are found in all regions of the stomach. These cells are columnar in shape, similar to the mucous surface cells. The apical cytoplasm is filled with a secretory product that is water-soluble and lubricates the stomach contents.

Parietal Cells

*Parietal cells* are large round or pyramid-shaped cells. They are very numerous in the proximal portions of the gastric glands. Parietal cells secrete intrinsic factor, a glycoprotein that helps absorb vitamin B12 across the intestinal lining. Parietal cells also secrete hydrochloric acid (HCl). Hydrochloric acid lowers the pH of the gastric juice, kills microorganisms, helps break down plant cell walls and connective tissues in meat, and denatures proteins.

The Jejunum

A sharp bend, the duodenojejunal flexure, marks the junction of the duodenum and the *jejunum*. At the junction the small intenstine re-enters the peritoneal cavity, becoming intraperitoneal and supported by a sheet of mesentery. the jejunum is about 8 feet long. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.

Histology of the Stomach

A simple columnar epithelium lines all regions of the stomach. The surface epithelium is composed of mucous surface cells. This secretory sheet of mucous surface cells produces a layer of mucus that covers the luminal surfaces of the stomach and protects the epithelium against acids and enzymes. Shallow depressions, called *gastric pits*, open onto the gastric surface. Regenerative stem cells are found at the base of each gastric pit. These cells actively divide to replace superficial cells that are shed continuously into chyme. The continual replacement of the superficial epithelial cells provides an additional defense against the gastric contents. If stomach acid and digestive enzymes penetrate the mucous layers, any damaged epithelial cells are quickly replaced.

Gastric Secretory Cells

Each gastric pit communicates with several *gastric glands* that extend deep into the underlying lamina propria. Gastric glands are simple branched tubular glands. Gastric pits and gastric glands contain four types of secretory cells: mucous neck cells, parietal cells, G cells, and chief cells. The parietal and chief cells work together to secrete about 1500 ml of *gastric juice* each day.

Histology of the Liver

Each lobe of the liver is divided by connective tissue into approximately 100,000 *liver lobules*, the basic functional units of the liver.

Histology of the Small Intestine

The histology of the small intestine follows the "typical" structure of a hollow organ of the digestive tract: mucosa, muscularis mucosae, submucosa, muscular layer, and serosa.

Masticatioin

The muscles of mastication close the jaws and slide or rock the lower jaw from side to side. During mastication, food is forced back and fourth between the vestibule and the rest of the oral cavity, crossing the recrossing the occlusal surfaces of the teeth. This movement of food results in part from the action if the masticatory muscles. However, control would be impossible without the aid of the buccal, labial, and lingual muscles. Once the food has been shredded or torn to a satisfactory consistency and moistened with salivary secretions, the tongue begins compacting the debris into a bolus that can be swallowed.

The Small Intestine

90% of nutrient absorption occurs in the *small intestine*, and most of the rest occurs in the proximal portion of the large intestine. The small intestine averages 6 m in length and has an average of 4 cm at its junction with the stomach and about 2.5 cm at its junction with the large intestine. The small intestine is found in all abdominal regions except the left hypochondriac and epigastric regions. The small intestine fill smuch of the peritoneal cavity. It is held in place by mesenteries attached to the dorsal body wall. The anatomy of the small intestine is specialized to increase its surface area for absorption and secretion. The intestinal lining has a series of ring-shaped projections called *circular folds*. Unlike the gastric folds in the stomach, each circular fold is a permanent feature of the intestinal lining. These folds do not disappear as the small intestine fills. Roughly 800 circular folds are found along the length of the duodenum, jejunum, and proximal half of the ileum. The mucosa possesses intestinal villi, and each cell of the surface epithelium has small microvilli on its apical surface.

The Serosa

Along regions of the digestive tract ithin the peritoneal cavity the muscular layer is covered by a serous membrane known as the *serosa*. There is no serosa, however, surrounding the muscular layer of the pharynx, esophagus, and rectum. Instead, the muscular layer is wrapped by a dense network of collagen fibers that firmly attaches the digestive tract to adjacent structures. This fibrous sheath is the *adventitia*.

Intestinal Glands

Between the columnar epithelial cells, goblet cells secrete mucin onto the intestinal surfaces. At the bases of the villi are the entrances to the *intestinal glands*. These glandular pockets extend deep into the underlying lamina propria. Near the base of each gland, stem cells continually divide, producing new generations of epithelial cells. These new cells move superficially toward the intestinal surface. They reach the top of the gland within a few days, renewing the epithelial surface and adding intracellular enzymes to the chyme. *Paneth cells* at the base of the intestinal glands have a role in innate (nonspecific) immunity and release defensins and lysozyme. These secretions kill some bacteria and allow others to live, thereby establishing the flora of the intestinal lumen. Intestinal glands also contain enteroendocrine cells that produce several intestinal hormones.

Peristalsis and Segmentation

Contractions within the muscularis mucosae that move a *bolus* (a small oval mass of food) along the digestive tract are called *peristalsis*. During a *peristaltic wave*, the circular muscle contract behind the digestive contents. Longitudinal muscles contract next, shortening adjacent segments. A wave of contraction in the circular muscles then forces the materials in the desired direction. Most areas of the small intestine and some regions of the large intestine undergo *segmentation*. These movements churn and fragment the digestive materials, mixing the contents with interstitial secretions. They do not move the bolus in any particular direction. Segmentation and peristaltic contractions are also triggered by hormones, chemicals, and physical stimulation. Afferent and efferent fibers of the glossopharyngeal, vagus, or pelvic nerves initiate peristaltic waves. Sensory receptors in the wall of the digestive tract trigger local peristaltic movements that are limited to a few centimeters of the digestive tract. These afferent fibers synapse within the myenteric neural plexus and produce localized, short *myenteric reflexes* that do not involve the CNS. The term enteric nervous system refers to the neural network coordinating these reflexes. In general, short reflexes control activities in one region of the digestive tract. This control involves coordinating local peristalsis and triggering the secretion of digestive glands. Many neurons are involved - the enteric nervous system has about as many neurons and neurotransmitters as the spinal cord has. Sensory information from receptors in the digestive tract is also distributed to the CNS, where it triggers long reflexes. Long reflexes involve interneurons and motor neurons in the CNS and provide a higher level of control over digestive and glandular activities. These reflexes generally control large-scale peristaltic waves that move materials from one region of the digestive tract to another. Long reflexes may involve motor fibers in the glossopharyngeal, vagus, or pelvic nerves that synapse in the myenteric neural plexus.

The Regulation of Pancreatic Secretion

Hormones released by the duodenum trigger the secretion of pancreatic juice. When acid chyme arrives in the small intestine, secretin is released. This hormone triggers the production of the watery pancreatic juice containing buffers. Another duodenal hormone, cholecystokinin, stimulates the production and secretion of pancreatic enzymes.

Regulation of the Large Intestine

Movement from the cecum to the transverse colon is very slow, allowing for water absorption to convert the already thick material into a sludgy paste. Peristaltic waves move material along the length of the colon. Segmentation movements, called *haustral churning*, mix the contents of adjacent haustra. When the stomach and duodenum are distended, signals are relayed over the intestinal neural plexuses. These signals cause powerful peristaltic contractions called *mass movements* that move material from the transverse colon through the rest of the large intestine. The contractions force feces into the rectum. The rectal chamber is typically empty, except when a mass movement forces feces out of the sigmoid colon into the rectum. When feces move into the rectum, the distension on the rectal wall stimulates stretch receptors, initiating the defecation reflex. The defecation reflex relaxes the internal anal sphincter, and feces move into the anal canal. When the external anal sphincter is voluntarily relaxed, defecation can occur.

The Oral Cavity

Our exploration of the digestive tract follows the path of food from the mouth to the anus. The mouth opens into the oral cavity. The functions of the oral cavity include (1) sensory analysis of food before swallowing, (2) mechanical digestion through the actions of the teeth, tongue, and palatal surfaces, (3) lubrication by mixing ingested materials with mucus and saliva, and (4) limited chemical digestion of carbohydrates by salivary amylase and lipids by lingual lipase.

Pancreatic Enzymes

Pancreatic enzymes are classified according to their intended targets. *Lipases* digest lipids; *carbohydrases*, such as pancreatic amylase, digest sugars and starches; *nucleases* break down nucleic acids; and *proteolytic enzymes* break proteins apart. The proteolytic enzymes include *proteinases* and *peptidases*. Proteinases break apart large protein complexes, and peptidases break small peptide chains into individual amino acids.

The Swallowing Process

Swallowing is a complex process that is initiated voluntarily but continues involuntarily. There are 3 phases of swallowing: 1) The *buccal phase* of swallowing starts when the bolus of food is compressed against the hard palate. The tongue retracts, which (a) forces the bolus into the pharynx, (b) helps the palatal muscles elevate the soft palate, and (c) isolates the nasopharynx. The buccal phase is voluntary. However, once the bolus enters the oropharynx, involuntary reflexes take over and move the bolus toward the stomach. 2) The *pharyngeal phase* begins when the bolus comes in contact with the palatal arches, the posterior pharyngeal wall, or both. The palatopharyngeus and stylopharyngeus muscles elevate the larynx. The folding of the epiglottis directs the bolus past the closed glottis. In less than a second, the pharyngeal constrictor muscles propel the bolus into the esophagus. As the bolus travels through the pharynx and into the esophagus, the respiratory centers are inhibited and breathing ceases. 3) The *esophageal phase* of swallowing start when the *upper esophageal sphincter* opens. After passing through the open sphincter, peristaltic waves push the bolus through the esophagus. As the bolus approaches the stomach, the *lower esophageal sphincter* opens, and the bolus enters the stomach.

The Stomach

The functions of the *stomach* are to (1) temporarily store and ingest food, (2) mechanically digest food, and (3) chemically digest food through the action of acids and enzymes. This mixing of ingested substances with the acids and enzymes secreted by the glands of the stomach produces a viscous, strongly acidic, soupy mixture called *chyme*.

The Lamina Propria

The lamina propria is the loose areolar connective tissue located within the core of each intestinal villus. The lamina propria contains numerous lymphatic cells, occasional lymphoid nodules, and an extensive network of capillaries that absorbs and carries nutrients to the hepatic portal circulation. Each villus also contains a central lymphatic vessel called a *lacteal*. Lacteals transport materials that cannot enter blood capillaries. For example, absorbed fatty acids are assembled into protein-lipid packages that are too large to diffuse into the bloodstream. These packets, called chylomicrons, reach the venous circulation through the thoracic duct.

The Intestinal Epithelium

The mucosa of the small intestine forms the fingerlike projections, *intestinal villi*, that project into the lumen. Each villus is covered by a simple columnar epithelium. The apical surfaces of the epithelial cells are covered with even smaller projections called *microvilli*. The circular folds, villi, and microvilli of the mucosa increase the surface area of the small intestine by a factor of more than 600 to approximately 2 million cm squared.

The Duodenum

The shortest and widest segment of the small intestine is the *duodenum*. It is approximately 10 inches long. The duodenum is connected to the pylorus of the stomach. A ring of smooth muscle, the pyloric sphincter, regulates movement of chyme from the stomach into the duodenum. From its start at the pyloric sphincter, the duodenum curves in a C that surrounds the pancreas. The proximal 1 inch portion is intraperitoneal, while the rest is secondarily retroperitoneal and located between vertebrae L1 and L4. The Duodenum is a "mixing bowl" that receives chyme from the stomach and digestive secretions from the pancreas and liver. Almost all essential digestive enzymes enter the small intestine from the pancreas.

Histological Organization of the Gallbladder

The wall of the gallbladder is composed of only the mucosa, lamina propria, muscular layer and serosa; a muscularis mucosae and submucosa are lacking. The mucosa has numerous folds that divide the surface into irregular *mucosal glands*. The lamina propria is composed of areolar connective tissue, and the muscular layer is composed of two interlacing layers of smooth muscle. The inner layer is composed mostly of longitudinally arranged smooth muscle, while the outer layer is composed mostly of circularly arranged smooth muscle.

Blood Supply to the Liver

Two blood vessels devlierblood to the liver, the *hepatic artery proper* and the *hepatic portal vein*. Roughly one-third of the normal hepatic blood flow arrives through the hepatic artery; the rest arrives through the hepatic portal vein. Blood returns to the systemic circuit through the *hepatic veins*, which empty into the inferior vena cava. The arterial supply provides oxygen-rich blood to the liver, and the hepatic portal vein supplies nutrients and other chemical absorbed from the intestine.

Regulation of Gastric Activity

Two mechanisms exert direct control over the production of acid and enzymes by the gastric mucosa. One regulatory mechanism involves the CNS and uses the vagus nerve (parasympathetic innervation) and branches of the celiac plexus (sympathetic innervation). For example, the sight or thought of food triggers motor output in the ganglionic parasympathetic fibers innervating parietal cells, chief cells, and mucous cells of the stomach. Stimulation increases the production of acids, enzymes, and mucus. The arrival of food in the stomach stimualtes stretch receptors in the stomach wall and chemoreceptors in the mucosa. Reflexive contractions occur in the muscular layers of the stomach wall, and G cells release gastrin. Both parietal and chief cells respond to the presence of gatrin by accelerating their secretory activities. Parietal cells are especially sensitive to gastrin, so the rate of acid production increases more dramatically than the rate of enzyme secretion. Sympathetic activation inhibits grastric activity. In addition, the small intestine releases two hormones that inhibit gastric secretion. *Secretin* and *cholecystokinin* stimulate secretion by both the pancreas and liver. The depression of gastric activity is a secondary, but complementary, effect.

The Colon

Several distinct features of the colon: - The wall of the colon forms a series of pouches, or *haustra*. The haustra permit considerable distension and elongation. Cutting into the intestinal lumen reveals that the creases between the haustra extend into the mucosal lining, producing a series of internal folds. - Three separate longitudinal band of smooth muscle- called the *teniae coli* - run along the outer surfaces of the colon just deep to the serosa. These bands correspond to the outer, longitudinal layer of the muscular layer in other portions of the digestive tract. Muscle tone within the teniae coli is what creates the haustra. - The serosa of the colon contains numerous teardrop-shaped sacs of fat called the *omental appendices*, or fatty appendices of the colon.

Histological Organization of the Digestive Tract

The major layers of the digestive tract are the (1) mucosa, (2) submucosa, (3) musvular layer, and (4) serosa. Variations in the structure of these four layers from organ to organ are related to the specific functions of each organ.

The Ileum

The third and last segment of the small intestine is the ileum. It is intraperitoneal and is the longest segment of the small intestine, averaging 12 feet in length. The ileum ends at the ileocecal valve. This sphincter controls the flow of material from the ileum into the cecum of the large intestine. The ileocecal valve protrudes into the cecum. There is no sharp anatomical distinction between the jejunum and the ileum. However, the following subtle anatomical differences enable a surgeon to distinguish the jejunum from the ileum: - The jejunum has a thicker wall and a larger diameter than the ileum. - The jejunum is typically found in the umbilical region, while the ileum tends to lie in the hypogastric region of the abdominal cavity. - The mesentery covering the ileum is typically thicker and contains more fat than does the mesentery covering the jejunum. - The vasculature extending from the mesentery to the jejunum tends to be more straightforward and has fewer branches than the vasculature extending from the mesentery to the ileum.

Histological Organization of the Pancreas

Connective tissue septa divide the pancreatic tissue into distinct lobules. The blood vessels and tributaries of the pancreatic ducts are found within these connective tissue septa. The pancreas is an example of a compound tubulo-acinar gland. The secretory units of the exocrine pancreas are the *pancreatic acini*. Each pancreatic acini are the beginning of the duct system of the pancreas. These ducts drain into progressively larger ducts, which drain into the main pancreatic duct, which runs the length of the pancreas. The pancreatic acini produce *pancreatic juice*, a mixture of water, ions, and pancreatic digestive enzymes. These enzymes are released into the duodenum, where they break down ingested materials into small molecules suitable for absorption. The pancreatic ducts also secrete buffers (primarily sodium bicarbonate). These buffers neutralize the acid in chyme and stabilize the pH of the intestinal contents. As we saw in Chapter 19, pancreatic islets are scattered between the acini. They account for approximately 1% of the cellular population of the pancreas and are responsible for the organ's endocrine function.

Aging and the Digestive System

Digestion and absorption are essentially normal in elderly individuals. However, many changes in the digestive system parallel age-related changes in other systems: - The effects of cumulative damage become apparent. A familiar example is the gradual loss of teeth because of tooth decay or gingivitis. Cumulative damage can involve internal organs as well. Toxins such as alcohol, heavy metals, and other chemicals that are absorbed by the digestive tract are transported to the liver for processing or storage. The liver cells are not immune to these compounds, and chronic exposure can lead to cirrhosis or other types of liver disease. - The rate of epithelial stem cell division declines. Stem cells in the epithelium divide less frequently with age, so tissue repair is less efficient. As a result, the digestive epithelium becomes more susceptible to damage by abrasion, acids, or enzymes. Peptic ulcers become more likely. In the mouth, esophagus, and anus, the stratified epithelium becomes thinner and more fragile. - Smooth muscle tone decreases, general motility decreases, and peristaltic contractions are weaker. These changes slow down the movement of chyme and promote constipation. Sagging in the walls of haustra in the colon produces symptoms of diverticulitis. Straining to eliminate compacted fecal materials stresses the less resilient walls of the blood vessels, producing hemorrhoids. A weakened cardiac sphincter can lead to esophageal reflux and frequent "heartburn." - Cancer rates increase. Cancers are most common in organs where stem cells divide to maintain epithelial cell populations. Rates of colon cancer and stomach cancer rise in the elderly; oral and pharyngeal cancers are particularly common in elderly smokers. - Changes in other systems affect the digestive system directly or indirectly. For example, reduced bone mass and calcium content in the skeleton result in the erosion of tooth sockets and eventual tooth loss. The decline in olfactory and gustatory sensitivity with age leads to dietary changes that affect the entire body.

Sets of Teeth

During development, two sets of teeth begin to form: deciduous and permanent. The *deciduous teeth* (falling off) appear first. Most children have 20 deciduous teeth. On each side of the upper or lower jaw, the deciduous teeth consist of two incisors, one canine, and a pair of deciduous molars for a total of 20 teeth. These teeth are later replaced by the *permanent teeth* of the larger adult jaws. As replacement proceeds, the periodontal ligaments and roots of the deciduous teeth erode. The deciduous teeth either fall out or are pushed aside by the *eruption*, or emergence, of the permanent teeth. The adult premolars take the place of the deciduous molars, while the adult molars extend the rows of teeth posteriorly and bring the permanent tooth count to 32. The third molars, or wisdom teeth, may not erupt before age 21. Wisdom teeth may not erupt because they develop in inappropriate positions of because there is not enough space.

Digestive Tract

Few of us think about our digestive system unless it malfunctions, yet each day we spend conscious effort filling and emptying it. References to this system are part of our everyday language. We may have a "gut feeling" or find an option "hard to swallow." A muscular tube called the *digestive tract* (or alimentary canal) and various *accessory organs* make up the digestive system. The digestive tract includes the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine. The accessory organs include the teeth, tongue, and various glandular organs, such as the salivary glands, liver, gallbladder, and pancreas. The accessory glandular organs secrete water, enzymes, buffers, and other components into ducts emptying into the digestive tract. As food enters and passes along the digestive tract, the secretions prepare nutrients for absorption across the digestive epithelium. The digestive tract and accessory organs work together to perform the following functions: - *Ingestion:* Ingestion occurs when foods and liquids enter the digestive tract via the mouth. - *Mechanical processing:* Most ingested solids undergo mechanical processing by the tongue and teeth before they are swallowed. Swirling, mixing, churning and propulsive motions of the digestive tract provide additional mechanical processing after swallowing. - *Digestion:* Digestion is the chemical and enzymatic breakdown of complex sugars, lipids, and proteins into small organic molecules that are absorbed by the digestive epithelium. - *Secretion:* Digestion involves the action of acids, enzymes, and buffers produced by active secretion. Some of these secretions are produced by accessory organs, such as the pancreas. - *Absorption:* Absorption is the movement of organic molecules, electrolytes, vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract. - *Excretion:* Excretion is the elimination from the body of the undigested residue of food and the waste products of metabolism. Waste products are secreted into the digestive tract, primarily bu the accessory glands, especially the liver. - *Compaction:* Compaction is the progressive dehydration of undigested materials and organic wastes prior to excretion from the body. The compacted material is called *feces*. *Defecation* is the elimination of feces from the body through the anus. The lining of the digestive tract plays an important role by protecting surrounding tissues against (1) the corrosive effects of digestive acids and enzymes, (2) mechanical stresses, such as abrasion, and (3) pathogens that are swallowed with food or reside within the digestive system. In summary, the organs of the digestive system mechanically and chemically process food that is eaten and passed along the digestive tract. These activities reduce the solid, complex chemical structures of food into small molecules. The epithelium lining the digestive tract absorbs these small molecules for transfer to the circulating blood.

Chief Cells

Found only in the fundus, *chief cells* are most abundant in the base of a gastric gland. These cells are columnar in shape and secrete *pepsinogen*, an inactive proenzyme. Acid in the gastric lumen converts pepsinogen to *pepsin*, an active proteolytic, or protein-digesting, enzyme. Pepsin functions most effectively in an acidic pH of 1.5-2. The stomachs of newborn infants (but not adults) produce *rennin* and *gastric lipase*, enzymes important for digesting milk. Rennin coagulates milk proteins, and gastric lipase initiates the digestion of milk fats. The lining of the stomach does not directly absorb any nutrients. However, some salts, water, and lipid-soluble substances are indirectly absorbed in the stomach. For instance, alcohol and some drugs, such as aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs), enter the lamina propria of the mucosa after damaging the surface epithelium. These substances then enter the bloodstream through capillaries within the lamina propria.

The Liver Lobule

In the liver lobule, the liver cells, or *hepatocytes*, form a series of irregular plates arranged like the spokes on a wheel. Up to the age of 7, the plates are no more than two cells thick. After age 7 the plates are only one cell thick. The exposed apical and basal surfaces of the hepatocytes are covered with short microvilli. Blood entering the liver by the hepatic artery proper and the hepatic portal system drain into highly fenested capillaries termed hepatic sinusoids that surround the plates of hepatocytes. These hepatic sinusoids empty into the *central vein*. The fenestrated walls of the sinusoids contian large openings that allow the substances to pass out of the circulation and into the spaces surrounding hepatocytes. The sinusoidal lining includes a large number of *stellate macrophages*. These phagocytic cells are part of the monocyte-macrophage system. They engulf pathogens, cell debris, and damaged blood cells. Stellate macrophages also store iron, some lipids, and heavy metals, such as tin or mercury, that are absorbed by the digestive tract. Blood enters the liver sinusoids from small branches of the portal vein and hepatic artery proper. A typical lobule is hexagonal (six-sided) in cross section. There are six *portal triads*, one at each corner of the lobule. A portal triad contains three structures: (1) an interlobular vein, (2) an interlobular artery, and (3) an interlobular bile duct. Branches from the arteries and veins deliver blood to the sinusoids of adjacent liver lobules. As blood flows through the sinusoids, hepatocytes absorb solutes form the plasma and secrete materials such as plasma proteins. Blood then leaves the sinusoids and enters the central vein of the lobule. the central veins ultimately merge to form the hepatic veins, which then empty into the inferior vena cava.

The Cecum and Appendix

Material arriving from the ileum first enters an expanded pouch called the *cecum*. The ileum attaches to the medial surface of the cecum and opens into the cecum at the *ileocecal valve*. The cecum collects and stores materials from the ileum and begins the process of compaction. The slender, hollow *appendix*, or vermiform appendix, is attached to the posteromedial surface of the cecum. The appendix is normally about 3.6 inches long, but its size and shape are quite variable. A small mesentery called the *meso-appendix* connects the appendix to the ileum and cecum. The primary function of the appendix is an organ of the lymphatic system. Inflammation of the appendix is known as appendicitis. Both the cecum and appendix are intraperitoneal.

Histology of the Large Intestine

Several histological characteristics distinguish the large intestine from the small intestine: - The wall of the large intestine is much thinner than that of the small intestine, even though the diameter of the colon is roughly three times that of the small intestine. - The large intestine lacks villi, which are characteristic of the small intestine. - Goblet cells are more abundant in the mucosal epithelium of the large intestine. - The intestinal glands of the large intestine are deeper than those of the small intestine, and they have more goblet cells. Secretion occurs as a result of local reflexes involving the local neural plexuses. This produces large amount of mucus, which lubricates the mucosa as undigested waste is compacted and passed along the large intestine. - Large lymphoid nodules are scattered throughout the lamina propria and extend into the submucosa. - The muscular layer differs from that of other intestinal regions. The longitudinal layer has been reduced to the three muscular bands of the teniae coli. However, the mixing and propulsive contractions of the colon resemble those of the small intestine.

The Gallbladder

The *gallbladder* is a hollow, pear-shaped muscular sac. The gallbladder stores and concentrates bile before it is released into the small intestine. The gallbladder is located in a recess, or fossa, in the posterior surface of the liver's right lobe. Like the liver, the gallbladder is intraperitoneal. The gallbladder is divided into three regions: the *fundus*, *body*, and *neck*. The *cystic duct* exits the gallbladder and unites with the common hepatic duct to create the *bile duct*. At the duodenum, a muscular *sphincter of ampulla* surrounds the lumen of the bile duct and the duodenal ampulla. The duodenal ampulla opens into the duodenum at the duodenal papilla, a small raised projection. Contraction of this sphincter seals off the passageway and prevents bile from entering the small intestine. The gallbladder has two major functions: storing and modifying bile. When the sphincter of the ampulla is closed, bile enters the cystic duct and flows into the gallbladder for storage. When filled to capacity, the gallbladder contains 40-70 ml of bile. As bile remains in the gallbladder, its composition gradually changes. Bile ejection occurs under stimulation of the hormone cholecystokinin (CCK). Cholecystokinin is released into the bloodstream at the duodenum when chyme arrives containing large amounts of lipids and partially digested proteins. CCK relaxes the sphincter of the ampulla and contracts the gallbladder.

The Liver

The *liver* is the largest visceral organ, a firm, reddish-brown organ weighing about 3.3 pounds. Most of its mass lies in the right hypochondriac and epigastric regions, by it may extend into the left hypochondriac and umbilical regions as well. The versatile liver performs essential metabolic and synthetic functions: - *Metabolic regulation:* The liver is an essential organ for regulating body metabolism. The liver regulates the circulating levels of carbohydrates, lipids, and amino acids. Blood draining all of the absorptive surfaces of the stomach and small and large intestines enters the hepatic portal system and flows into the liver. As a result, the liver cells extract absorbed nutrients and toxins fro the blood before they reach the systemic circulation through the hepatic veins. Liver cells (called hepatocytes) monitor circulating levels of metabolites and make any necessary adjustments. Excess nutrients are removed and stored. Deficiencies are corrected by mobilizing stored reserves or performing appropriate synthetic activities. Circulating toxins and metabolic wastes are also removed. They are then inactivated, stored, or excreted. Finally, fat-soluble vitamins (A, D, K, and E) are absorbed and stored in the liver. - *Hematological regulation:* The liver is the largest blood reservoir in the body, receiving about 25% of the cardiac output. As blood passes through the liver phagocytic cells remove old or damaged RBCs, cellular debris, and pathogens. Liver cells synthesize and secrete plasma proteins into the blood. Plasma proteins contribute to the osmotic concentration of the blood, transport nutrients, and establish the clotting and complement systems. - *Production of bile:* Bile is synthesized by liver cells, stores in the gallbladder, and excreted into the lumen of the duodenum. Bile is mostly water, with small amount of ions, bilirubin (a pigment derived from hemoglobin), and an assortment of lipids known as bile salts. The water and ions dilute and buffer acids in the chyme as it enters the small intestine. Bile salts make it possible for enzymes to break down lipids in the chyme into fatty acids suitable for absorption. The liver is thought to have more than 200 functions. Any condition that severely damaged the liver presents a serious threat to life. Liver cells do have a limited ability to regenerate after an injury. However, liver function will not fully recover unless the normal vascular patterns also regenerate.

The Mucosa

The *mucosa*, the inner lining of the digestive tract, is a *mucous membrane*, which is a layer of loose connective tissue covered by an epithelium moistened by glandular secretions. The *mucosal epithelium* is either stratified or simple epithelium, depending on the location and the stresses involved. For example, the oral cavity and esophagus are lined by a nonkeritanized stratified squamous epithelium that resists stress and abrasion. In contrast, the stomach, small intestine, and almost the entire large intestine encounter considerably less stress and abrasion than the oral cavity and esophagus. As a result, these structures have a simple columnar epithelium specialized for secretion and absorption. Many segments of the digestive tract have *circular folds* or plicae circulares. These transverse or longitudinal folds of the mucosa increase the surface area available for absorption and secretion. In some regions of the digestive tract, circular folds are permanent and features involving both the mucosa and submucosa. In other regions, circular folds are temporary features and disappear as the lumen fills, enabling the lumen to expand after a large meal. Ducts opening onto the epithelial surfaces carry the secretions of gland cells located either in the mucosa and submucosa or within accessory organs. A layer of areolar connective tissue is found deep to the epithelium of the mucosa. This layer, the *lamina propria*, contains blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle fibers, and scattered areas of lymphatic tissue. In most regions of the digestive tract the border of the mucosa is a narrow band of smooth muscle and elastic fibers. This band of smooth muscle is called the *muscularis mucosae*. The smooth muscle fibers in the muscularis mucosae are arranged in two thin concentric layers - an internal circular layer and an external longitudinal layer. Contraction of these layers alters the shape of the lumen and moves the epithelial pleats and folds.

The Muscular Layer

The *muscular layer* is a double layer of smooth muscle fibers deep to the submucosa. These smooth muscle fibers are arranged in internal circular and external longitudinal layers. These layers of smooth muscle mechanically process and propel materials along the digestive tract. These movements are coordinated by neurons of the *myenteric neural plexus*. This plexus, located between the two muscular layers, is composed of parasympathetic ganglia and sympathetic postganglionic fibers. Parasympathetic stimulation increases muscular tone and stimulates contractions, while sympathetic stimulation decreases muscular tone and promotes relaxation. At specifc locations along the digestive tract there are thickened areas of the muscular circular layer. These localized thickenings form *sphincters*, or valves. The sphincters constrict to restrict movement or to ensure one-way passage of materials along the lumen.

Anatomy of the Oral Cavity

The *oral mucosa* lines the *oral cavity*. The oral mucosa is a nonkeritanized stratified squamous epithelium that protects the mouth from abrasion. Buccal fat pads and the buccinator support the mucosa of the *cheeks*, which are the lateral walls of the oral cavity. Anteriorly, the mucosa of the cheeks is continuous with the *lips*. The space between the cheeks, lips, and teeth is the *oral vestibule*. The ridges of oral muscosa, the *gingivae*, or gums, surround the base of each tooth on the alveolar processes of the maxillae and the alveolar part of the manbidle. The tongue forms the floor of the oral cavity. The mylohyoid, which is located inferior to the tongue, provides additional support to the floor of the oral cavity. The *hard palate* separates the oral cavity from the nasal cavity. The hard palate is formed by the palatine processes of the maxilla and the horizontal plates of the palatine bones. The *soft palate* lies posterior to the hard palate. It separates the oral cavity from the nasopharynx and closes off the nasopharynx during swallowing. The finger-shaped *uvula* dangles from the center of the posterior margin of the soft palate. The uvula prevents food from entering the pharynx too soon. The two pairs of muscular *palatal arches* are found on each side of the uvula. 1) The Palatoglossal arches extend between the soft palate and the base of the tongue. Each arch consists of a mucous membrane and the underlying palatoglossus muscle and associated tissues. 2) The palatopharyngeal arches extend from the soft palate to the side of the pharynx. Each arch consists of a mucous membrane and the underlying palatopharyngeus muscle and associated tissues. The palatine tonsils lie between the palatoglossal and palatopharyngeal arches. The space between the oral cavity and the pharynx, bounded by the soft palate and the base of the tongue, is called the *fauces*.

The Submucosa

The *submucosa* is a layer of areolar connective tissue deep to the muscularis mucosae. Large blood and lymphatic vessels are found in this layer. In some regions of the digestive tract the submusoca also contains exocrine glands that secrete buffers and enzymes into the lumen. The submucosa contains a network of nerve fibers and scattered neuron cell bodies. These *submucosal neural plexuses* innervate the mucosa. They contain sensory neurons, parasympathetic ganglia, and sympathetic postganglionic fibers.

The Tongue

The *tongue* has four primary functions: (1) mechanical digestion by compression, abrasion, and distortion, (2) manipulation to assist in chewing and to prepare food for swallowing, (3) sensory analysis by touch, temperature, and taste receptors, and (4) secretion of mucin and the enzyme lingual lipase. The tongue is divided into an anterior *body* and a posterior *root*. The *dorsum of the tongue* contains numerous fine projections called lingual papillae. Each papillae has a thick layer of stratified squamous epithelium, which produces additional friction that helps the tongue move materials around. Additionally, taste buds are found along the edges of many papillae. A V-shaped line of circumvallate papillae roughly indicates the boundary between the body and the root of the tongue, which is situated in the pharynx. The tongue's epithelium is flushed by water, mucin, and lingual lipase, which are secretions form small glands extending into the lamina propria. Lingual lipase begins the enzymatic breakdown of lipids, specifically triglycerides. The epithelium covering the inferior surface of the tongue is thinner and more delicate than that of the dorsum. Alogn the inferior midline is the *frenulum of the tongue*. This thin fold of mucous membrane connects the body of the tongue to the mucosa of the oral floor. Ducts from two pairs of salivary glands open on each side of the frenulum, which prevents extreme movements of the tongue. However, an overly restrictive frenulum hinders normal eating or speech. This condition, called ankyloglossia, can be corrected surgically. The tongue contains two different groups of muscles, *extrinsic tongue muscles* and *intrinsic tongue muscles*. The hypoglossal nerve (XII) controls these muscles. The extrinsic muscles include the hyoglossus, styloglossus, genioglossus, and palatoglossus muscles. These muscles perform all gross movements of the tongue. The smaller intrinsic muscles change the shape of the tongue and assist the extrinsic muscles during precise movements, such as speech.

Accessory Digestive Organs

The accessory digestive organs are the lover, gallbladder, and pancreas. The organs produce and store enzymes and buffers essential for normal digestive functions. In addition to their roles in digestion, the liver and pancreas have exocrine functions.

Types of Teeth

The alveolar processes of the maxillae and the alveolar part of the mandible form the *maxillary dental arcade* and *mandibular dental arcade*, or upper and lower dental arcades, respectively. These arcades contain four types of teeth, each with specific functions. 1) *Incisors* are blade-shaped teeth found at the front of the mouth. Incisors clip and cut food - think of biting off the tip of a carrot stick. 2) The *canines*, or cuspids, are conical teeth with a sharp ridge-line and a pointed tip. They tear and slash food. We might bite off a tough piece of celery with our incisors, but we will then move it to one side to take advantage of the shearing action of the canines. Incisors and canines each have a single root. 3) *Premolars*, or bicuspids, have flattened crowns with two prominent rounded cusps. They crush, mash, and grind. Premolars have one or two roots. 4) *Molars* have very large, flattened crowns with four to five prominent rounded cusps adapted for crushing and grinding. Molars in the upper jaw typically have three roots, whereas those in the lower jaw usually have two roots.

Regulation of Salivary Sectrion

The autonomic nervous system controls the salivary gland secretions. Each salivary gland receives parasympathetic and sympathetic innervation. Food in the mouth triggers a salivary reflex by stimulating receptors monitored by the trigeminal nerve or by stimulating taste buds innervated by N VII, N IX, or N X. Parasympathetic stimulation speeds up secretion by all the salivary glands. As a result, you produce large amounts of saliva. The role of sympathetic innervation is unclear. Evidence suggests that it provokes the secretion of small amounts of very thick saliva.

Regions of the Colon

The colon is subdivided into four regions: the ascending colon, transverse colon, descending colon, and sigmoid colon. *Ascending Colon* The ascending colon begins at the superior border of the cecum. It ascends along the right lateral and posterior abdominal wall of the peritoneal cavity to the inferior surface of the liver. At this point the colon turns to the left at the *right colic flexure*. This bend marks the end of the ascending colon and the beginning of the transverse colon. The ascending colon is secondarily retroperitoneal; visceral peritoneum covers its lateral and anterior surfaces. *Transverse Colon* The transverse colon curves anteriorly from the right colic flexure and crosses the abdomen from right to left. As the transverse colon crosses the abdominal cavity, its peritoneal relationship changes. The initial segment is intraperitoneal. It is supported by the transverse mesocolon and separated from the anterior abdominal wall by the layers of the greater omentum. As the transverse colon passes inferior to the greater curvature of the stomach it becomes secondarily retroperitoneal. The gastrocolic ligament attaches the transverse colon to the greater curvature of the stomach. Near the spleen, the colon makes a right turn, termed the *left colic flexure*. It then proceeds caudally as the descending colon. *Descending Colon* The descending colon, which is secondarily retroperitoneal, proceeds inferiorly along the left side of the abdomen. At the iliac fossa, the descending colon curves and becomes the sigmoid colon. *Sigmoid Colon* The sigmoid colon is an S-shaped segment that is only about 6 inches long. The sigmoid colon lies posterior to the urinary bladder, suspended from the sigmoid mesocolon. The sigmod colon empties into the rectum.

Supporting of the Small Intestine

The duodenum has no supporting mesentery. The proximal 2.5 cm is intraperitoneal and movable, but the rest is secondarily retroperitoneal and fixed in position. The jejunum and ileum are intraperitoneal and supported by an extensive, fan-shaped mesentery - the mesentery proper. Blood vessels, lymphatic vessels, and nerves pass through the connective tissue of the mesentery to reach these segments of the small intestine. These blood vessels are branches of the superior mesenteric artery and tributaries of the superior mesenteric vein. The vagus nerve provides parasympathetic innervation; the superior mesenteric ganglion provides sympathetic innervation.

Regional Specializations

The duodenum, jejunum, and ileum of the small intestine each have logical specializations related to their primary functions. *The Duodenum* In addition to the intestinal glands, the submucosa contains *duodenal submucosal glands*, also known as Brunner's glands. These glands produce large quantities of mucus when chyme arrives from the stomach. The mucus protects the epithelium from the acidity of chyme and also contains bicarbonate ions that help raise the pH of chyme. Submucosal glands ar emost abundant in the proximal portion of the duodenum, and they decrease in numbers as you approach the jejunum. The pH of the intestinal contents changes from 1-2 to 7-8 as chyme moves from the duodenum to the jejunum. By the beginning of the jejunum, the extra mucus production is no longer needed. Buffers and enzymes from the pancreas and bile from the liver enter the duodenum roughly halfway along its length. The bile duct from the liver and gallbladder and the pancreatic duct from the pancreas come together within the duodenal wall at a muscular chamber called the *duodenal ampulla*. The duodenal ampulla opens into a small mound within the duodenum termed the *duodenal papilla*. The muscular *hepatopancreatic sphincter* encircles the lumen of the bile duct and, generally, the pancreatic duct and duodenal ampulla as well. *Jejunum and Ileum* Circular folds and villi are prominent in the proximal half of the jejunum. Most nutrient absorption occurs here. As you approach the end of the ileum, the folds and villi diminish in size and number. This reduction parallels the reduction in absorptive activity; most nutrient absorption has occurred before materials reach the ileum. The distal portions of the ileum lack circular folds. The lamina propria there contains 20-30 masses of lymphoid tissue called submucous *aggregated lymphoid nodules*, or Peyer's patches. These lymphoid tissues are most abundant in the terminal portion of the ileum, near the entrance to the large intestine. The lymphocytes in the aggregated lymphoid nodules protect the small intestine from bacteria that normally inhabit the large intestine.

Bile Secretion and Transport

The hepatocytes form bile. It is secreted into a network of narrow channels between adjacent liver cells. These small channels, called *bile canaliculi*, extend outward, away from the central vein. The canaliculi connect with fine *bile ductules* that carry bile to an *interlobular bile duct* in the nearest portal triad. the *right* and *left hepatic ducts* collect bile from all the bile ducts of the liver lobes. These ducts unite to form the *common hepatic duct* that leaves the liver. The bile in the common hepatic duct either flows into the bile duct that empties into the duodenal ampulla or enters the cystic duct that leads to the gallbladder.

The Large Intestine

The horseshoe-shaped *large intestine* begins at the junction with the ileum and ends at the anus. The large intestine lies inferior to the stomach and liver and forms an almost complete frame around the small intestine. The large intestine is often called the *large bowel*. It as an average length of about 5 feet and width of 3 inches. It is divided into 3 parts: (1) the cecum, (2) the colon, and (3) the rectum. The large intestine (1) absorbs water and electrolytes and compacts the intestinal contents into feces, (2) absorbs the important vitamins produced by bacteria, and (3) stores feces before defecation. The large intestine receives blood from branches of the superior mesenteric and inferior mesenteric arteries. Venous blood is collected from the large intestine by the superior mesenteric and inferior mesenteric veins.

Anatomy of the Liver

The liver is intraperitoneal. Deep to its visceral peritoneal layer is a tough fibrous capsule. On the anterior surface a central mesentery, the *falciform ligament*, marks the division between the *left lobe* and the *right lobe* of the liver. The *round ligament* is a thickening of the inferior margin of the falciform ligament. This fibrous band marks the path of the degenerated fetal umbilical vein. The liver is suspended form the inferior surface of the diaphragm by the *coronary ligament*. The shape of the liver conforms to its surroundings. Its *anterior surface* follows the smooth curve of the body wall. The *posterior surface* has impressions from the stomach, small intestine, right kidney, and large intestine. The superior, anterior, and posterior surfaces of the liver are called the diaphragmatic surfaces because of their anatomical relationships to the diaphragm. The inferior surface is called the visceral surface. The liver has classically been described as having four lobes. The impression left by the inferior vena cava marks the division between the right lobe and the small *caudate lobe*. Inferior to the caudate lobe lies the *quadrate lobe*, sandwiched between the left lobe and the gallbladder. The classical description of four lobes was based on the superficial topography of the liver. However, this description of the liver did not meet the needs of modern medical science, especially surgery. As a result, a more comprehensive system for describing the structure of the liver was developed. The new terminology subdivided the lobes of the liver into segments based on the major subdivisions of the hepatic artery, portal vein, and hepatic ducts. The actual boundaries cannot be determined without dissecting the liver.

The Teeth

The movements of the tongue pass food across the surfaces of the teeth. Teeth perform chewing, or *mastication*, of food. Mastication breaks down touch connective tissues in meat and plant fibers in vegetables. It also saturates food with salivary secretions and enzymes. Most of each tooth consists of *dentine*, or dentin, a mineralized matrix similar to bone. However, unlike bone, dentine does not contain living cells. Instead, cells in the central *pulp cavity* extend cytoplasmic processes into the dentine. The pulp cavity is spongy and highly vascular. Blood vessels and nerves enter the pulp cavity by a narrow tunnel, the *root canal*, located at the base, or *root*, of the tooth. The *dental artery*, *dental vein*, and *dental nerve* enter the root canal through the *apical foramen* at the tip of the root. The root of tooth sits in a bony cavity or socket called the *tooth socket*, or *tooth alveolus*. Collagen fibers of the *periodontal ligament* extend from the dentine of the root to the bone, creating a strong, fibrous articulation known as a gomphosis. A layer of *cement* covers the dentine of the root. This protects and firmly anchors the periodontal ligament. The cement is histologically similar to bone and less resistant to erosion than dentine. The *neck* of the tooth marks the boundary between the root and the *crown*, the visible portion of the tooth projecting above the soft tissue of the gingiva. A shallow groove called the *gingival sulcus* surrounds the neck of each tooth. This prevents bacteria from accessing the lamina propria of the gingiva or the relatively soft cement of the root. If this attachment breaks down, bacteria can infect the gingiva, causing inflammation termed gingivitis. A layer of *enamel* covers the dentine of the crown. Enamel forms the *occlusal surface* of each tooth, which is the biting surface that grinds food against the opposing tooth surface. Elevations or projections of the occlusal surface are called *cusps*. Enamel contains densely packed crystals of calcium phosphate and is the hardest biologically manufactured substance. Adequate amounts of calcium, phosphates, and vitamin D are essential during childhood if the enamel coating is to be complete and resistant to decay.

Muscular Layers and the Movement of Digestive Materials

The muscularis mucosae and muscular layer fo the digestive tract contain *smooth muscle*. Smooth muscle cells range from 5-7 um in diameter and from 30-200 um in length. They are surrounded by connective tissue, but unlike skeletal muscle, the collagen fibers do not form tendons or aponeuroses. In addition, unlike skeletal muscle cells, the contractile proteins of these smooth muscle cells are not organized into sarcomeres. Therefore, the smooth muscle cells lack striations. Although nonstriated, their contractions are as strong as those of skeletal or cardiac muscle cells. Because the contractile filaments of smooth muscle cells are not rigidly organized, a stretched smooth muscle cell soon adapts to its new length and still retains the ability to contract on demand. This ability to tolerate extreme stretching is called plasticity. Plasticity is especially important for digestive organs that undergo great changes in volume, such as the stomach. The smooth muscle cells of the digestive tract are involuntary muscle cells, and many have no autonomic nervous system motor innervation. *Pacesetter cells* are located in the muscularis mucosae and muscular layer that surround the lumen of the digestive tract. Pacesetter cells undergo spontaneous depolarization, triggering contraction of the smooth muscle. Gap junctions electrically connect the adjacent muscle cells. When one smooth muscle contracts, the contraction spreads in a wave that travels throughout the tissue. The initial stimulus may be a pacesetter cell or a motor neuron stimulation one of the smooth muscle cells in that region. It may also be a local response to chemicals, hormones, concentrations of oxygen or carbon dioxide, or physical factors such as extreme stretching or irritation.

Musculature of the Stomach

The muscularis mucosae and muscular layer of the stomach contain extra layers of smooth muscle cells in addition to the usual circular and longitudinal layers. The muscularis mucosae generally contain an outer, circular layer of muscle cells. The muscular layer has an inner *oblique layer* of smooth muscle. The extra layers of smooth muscle strengthen the stomach wall and assist in mixing and churning essential to the formation of chyme.

The Pancreas

The pancreas lies posterior to the stomach, extending laterally from the duodenum toward the spleen. The pancreas is an elongated, pinkish-gray organ, approximately 6 inches long and weighting around 3 ounces. It has three subdivisions: head, body, and tail. The broad *head* of the pancreas lies within the loop formed by the initial segment of the duodenum. The slender *body* extends transversely to the left toward the spleen, and the *tail* is short and rounded. The pancreas is secondarily retroperitoneal, and it is firmly bound to the posterior wall of the abdominal cavity. The surface of the pancreas has a lumpy, nodular texture. A thin, transparent connective tissue capsule wraps the pancreas. The pancreatic lobules, associated with blood vessels, and excretory ducts are easily seen through the anterior capsule and the overlying layer of peritoneum. The pancreas is a mixed gland, with both exocrine and endocrine functions. The exocrine portion of the pancreas produces digestive enzymes and buffers. The large *pancreatic duct* delivers its exocrine secretions to the duodenal ampulla. Sometimes a small *accessory pancreatic duct* branches from the pancreatic duct before it leaves the pancreas. The pancreatic duct exends within the attached mesentery to reach the duodenum, where it meets the bile duct from the liver and gallbladder. The two ducts then empty into the duodenal ampulla, a chamber located roughly halfway along the length of the duodenum. When present, the accessory pancreatic duct usually empties into the duodenum at a separate duodenal papilla, outside the duodenal ampulla. The splenic, superior mesenteric, and common hepatic arteries supply the pancreas with blood. The *pancreatic arteries* and *pancreaticoduodenal arteries* (superior and inferior) are the major branches from these vessels. The splenic vein and its branches drain the pancreas.

The Peritoneum

The peritoneum is a serous membrane with two parts. The *visceral peritoneum*, or serosa, is continuous with the *parietal peritoneum* lining the inner surfaces of the body wall. The organs of the abdominal cavity are often described as lying within the abdominal and peritoneal cavities. Abdominal organs demonstrate one or more of the following relationships with the peritoneal membranes: - *Intraperitoneal* organs lie within the peritoneal cavity, in that they are covered on all sides by the visceral peritoneum. The stomach, liver, and ileum are intraperioneal organs. - *Retroperitoneal* organs are covered by the parietal peritoneum on their anterior surface only, so they lie outside the peritoneal cavity. These organs typically do not develop from the embryonic gut. The kidneys, ureters, and abdominal aorta are retroperitoneal. - *Secondarily retroperitoneal* organs are organs of the digestive tract that form as intraperitoneal organs bu become retroperitoneal. The shift occurs during embryonic development as a portion of the visceral peritoneum fuses with the opposing parietal peritoneum. The pancreas and the distal two-thirds of the duodenum are secondarily retroperitoneal organs. The peritoneum is a serosal membrane. It continually produces a watery fluid that lubricates the peritoneal surfaces. About 7 liters of fluid are secreted and reabsorbed each day. However, the volume within the peritoneal cavity at any one time is very small. Under unusual conditions, such as liver disease, heart failure, or electrolyte imbalance, the volume of peritoneal fluid increases markedly, resulting in a dangerous reduction in blood volume and distortion of visceral organs.

The Pharynx

The pharynx serves as a common passageway for food, liquids, and air. Chapter 24 described the epithelial lining and divisions of the pharynx: the nasopharynx, oropharynx, and laryngopharynx. Deep to the lamina propria of the mucosa is a dense layer of elastic fibers, bound to the underlying skeletal muscles. Chapter 10 discussed the specific pharyngeal muscles involved in swallowing, which we summarize here: - The superior, middle, and inferior pharyngeal constrictors push the bolus toward the esophagus. - The palatopharyngeus and stylopharyngeus muscles elevate the larynx. - The palatal muscles raise the soft palate and adjacent portions of the pharyngeal wall. The pharyngeal muscles cooperate with muscles of the oral cavity and esophagus to initiate the swallowing process, or *deglutition*.

The Rectum

The rectum forms the last 6 inches of the digestive tract. It is an expandable organ for the temporary storage of *feces*. Movement of fecal material into the rectum triggers the urge to defecate. The last portion of the rectum is the *anal canal*. It contains small longitudinal folds, the *anal columns*. The distal margins of these columns are joined by transverse folds that mark the boundary between the columnar epithelium of the proximal rectum and a stratified squamous epithelium like that in the oral cavity. The *anus* is the exit of the anal canal. There, the epidermis becomes keratinized and identical to the surface of the skin. The circular layer of muscle in this region forms the *internal anal sphincter*. The smooth muscle fiberes in this sphincter are not under voluntary control. The *external anal sphincter*, which guards the anus, encircles the distal portion of the anal canal. This sphincter, which consists of a ring of skeletal muscle fibers, is under voluntary control. The lamina propria and submucosa of the anal canal contain a network of veins. If venous pressures there rise too high due to straining during defecation or pregnancy, the veins can become distended, producing hemorrhoids.

Anatomy of the Stomach

The stomach is intraperitoneal and shaped like an expanded letter J. The *anterior* and *posterior surfaces* are smooth and rounded. The stomach typically extends between the levels of vertebra T7 and L3. It occupies the left hypochondriac, epigastric, and portions of the umbilical and left lumbar regions. The shape and size of the stomach are extremely variable from individual to individual and from one meal to the next. The stomach is divided into four regions: 1) The *cardia* - so named because of its proximity to the heart - is the superior, medial portion of the stomach immediately surrounding its junction with the esophagus. The esophageal lumen opens into the cardia at the *cardiac orifice*. 2) The fundus is the region of the stomach that projects superior to the junction between the esophagus and the stoamch. The fundus contacts the inferior and posterior surfaces of the diaphragm. 3) The *body* of the stomach is the area between the fundus and the curve of the J. The largest region of the stomach, the body, functions as a mixing tank for ingested food and gastric secretions. 4) The *pyloric part* forms the portion of the stomach between the body of the stomach and the duodenum (first segment of the small intestine). It is divided into a *pyloric antrum*, which is connected to the body, a *pyloric canal*, which empties into the duodenum, and the *pylorus*, which is the muscular tissue surrounding the pyloric orifice (stomach outlet). During digestion, the shape of the pyloric part changes often. A thickening of the circular layer of the muscle within the pylorus, called the *pyloric sphincter*, regulates the release of chyme into the duodenum. The volume of the stomach increases at mealtimes and decreases as chyme leaves the stomach and enters the small intestine. When the stomach is empty, *gastric folds*, or gastric rugae (wrinkles) appear. As the stomach fills, the gastric folds gradually flatten out until, at maximum distension, they almost disappear.

Blood Supply to the Stomach

The three branches of the celiac trunk supply blood to the stomach: 1) The left gastric artery supplies blood to the lesser curvature and cardia. 2) The splenic artery supplies the fundus directly and the greater curvature through the left gastro-epiphloic artery. 3) The common hepatic artery branches into the right gastric, the right gastro-epiploic, and the gastroduodenal arteries. These vessels supply blood to the greater and lesser curvatures at the pylorus of the stomach. The gastric and gastro-epiploic veins drain blood from the stomach into the hepatic portal vein.

Histology of the Esophageal Wall

The wall of the esophagus has mucosa, submucosa, and muscular layers comparable to those described earlier. The esophageal wall has several distinctive features: - The epithelium of the mucosa is an abrasion-resistant nonkeritanized (mucosal), stratified squamous epithelium. - The mucosa and submucosa form large folds that run the length of the esophagus. These folds allow it to expand when a large bolus passes through. Muscle tone in the walls of the esophagus keeps the esophagus closed except during swallowing. - The smooth muscle layer of the muscularis mucosae is very thin or absent near the pharynx. As you more inferiorly within the esophagus, it gradually thickens to 200-400 um near the junction with the stomach. The muscularis mucosae of the esophagus is composed of only a single layer of longitudinal smooth muscle, which is different from the rest of the digestive tract. - The submucosa contains scattered esophageal glands. (*The esophagus is only one of two regions of the digestive tract that contains submucosal glands; the other is the duodenum.*) These simple, branched, tubular glands produce a mucous secretion that lubricates the bolus and protects the epithelial surface. - The muscular layer has inner circular and outer longitudinal muscle layers. In the superior third of the esophagus, both layers contain mostly skeletal muscle fibers and some isolated smooth muscle cells. In the middle third, there is an even mixture of skeletal and smooth muscle tissue. The inferior third contains only smooth muscle cells. Visceral reflexes control the skeletal muscle and smooth muscle in the esophagus; you do not have voluntary control over these contractions. - An adventitia of connective tissue outside the muscular layer anchors the esophagus to the posterior body wall. Between the diaphragm and stomach, the esophagus is retroperitoneal. Peritoneum covers the anterior and left lateral surfaces.

Salivary Glands

Three pairs of salivary glands secrete into the oral cavity: 1) The *parotid glands* are the largest salivary glands, weighing approx 20 g. The superior, anterior portion of each parotid gland extends between the inferior surface of the zygomatic arch and the anterior margin of the sternocleidomastoid. The posterior portion extends from the mastoid process of the temporal bone anteriorly, crossing the superficial surface of the masseter. The secretions of each gland are drained by a *parotid duct*, which empties into the oral vestibule at the second upper molar. 2) The mucous membrane of the floor of the mouth covers the sublingual glands*. Numerous *sublingual ducts* open along either side of the frenulum of the tongue. 3) The *submandibular glands* are found in the floor of the mouth along the medial surfaces of the mandible inferior of the myohyloid line. The *submandibular ducts* open into the mouth on either side of hte frenulum of the tongue, immediately posterior to the teeth. The saliva in the mouth is a mixture of all the salivary glands' secretions. About 70% of the saliva originates in the submandibular salivary glands, 25% in the parotid salivary glands, and 5% in the sublingual salivary glands. Approx 1-1.5 L of saliva is produced each day. It has a composition of 99.4% water, with the remainder being an assortment of ions, buffers, metabolites, and enzymes. Glycoproteins calle dmucin give saliva its lubricating action. At mealtimes the production of large quantities of saliva lubricates the mouth, moistens the food, and dissolves chemicals that stimulate the taste buds. A continual background level of secretion flushes the oral surfaces and controls the populations of oral bacteria. Reduction or elimination of salivary secretions triggers a bacterial population explosion in the oral cavity. This proliferation rapidly leads to recurring infections and the progressive erosion of teeth and gums.

The Esophagus

Tje *esophagus* is a hollow muscular tube that transports foods and liquids to the stomach. It lies posterior to the trachea slightly left of midline. It passes along the dorsal wall of the mediastinum in the thoracic cavity and enters the peritoneal cavity through an opening in the diaphragm, the *esophageal hiatus*. The esophagus empties into the stomach. The esophagus begins at the level of the cricoid cartilage anterior to vertebra C6 and ends anterior to vertebra T7. The esophagus receives blood from (1) the esophageal arteries, branches of (2) the thyrocervical trunk and (3) ecternal carotid arteries of the neck, (4) the bronchial arteries adn (5) the esophageal arteries if the mediastinum, (6) the inferior phrenic artery, and (7) the left gastric artery if the abdomen. Venous blood from the esophagus drains into the esophageal, inferior thyroid, azygos, and gastric veins. The vagus and sympathetic trunks innervate the esophagus via the esophageal plexus. Neither the upper not the lower portion of the esophagus has a well defined sphincter muscle comparable to those located elsewhere along the digestive tract. Nevertheless, the terms upper esophageal sphincter and lower esophageal sphincter (or cardiac sphincter) are used to describe these regions because they are similar in function to other sphincters.

Regulation of the Small Intestine

Weak peristaltic contractions slowly move materials along the length of the small intestine as absorption occurs. The movements of the small intestine are controlled by neural reflexes involving the submucosal and myenteric neural plexuses. Stimulation of the parasympathetic system increases the sensitivity of these reflexes and accelerates peristaltic contractions and segmentation movements. These contractions and movements promote the mixing of the intestinal contents and usually occur within a few centimeters of the original stimulus. When food enters the stomach, coordinated intestinal movements occur. These contractions move chyme away from the duodenum and toward the large intestine. At the same time, the ileocecal valve opens, allowing the passage of material into the large intestine.

Mesenteries

Within the peritoneal cavity, most regions of the digestive tract are suspended by sheets of serous membrane connecting the parietal peritoneum with the visceral peritoneum. These *mesenteries* are fused double sheets of peritoneal membrane. The areolar connective tissue between the two surfaces provides a route for the passage of blood vessels, nerves, and lymphatic vessels to and from the digestive tract. Mesenteries also stabilize the positions of the attached organs and prevent their entanglement during digestive movements or sudden changes in body position. During development, dorsal and ventral mesenteries suspend the digestive tract and accessory organs within the peritoneal cavity. The ventral mesentery later disappears along most of the digestive tract, remaining only in two locations. One is on the ventral surface of the stomach, between the stomach and the liver, from the *lesser omentum*. The second is between the liver and the anterior abdominal wall and diaphragm, forming the falciform ligament. (Although this peritoneal sheet is called a "ligament," it is not comparable to the ligaments interconnecting bones.) As the digestive system elongates, it twists and turns within the crowded peritoneal cavity. The dorsal mesentery of the stomach enlarges, forming a pouch extending inferiorly between the body wall and the anterior surface of the small intestine. This pouch is the *greater omentum*. The loose connective tissue within the mesentery of the greater omentum contains a thick layer of adipose tissue. The lipids in the adipose tissue are thought to have two possible functions: (1) serving as an important energy reserve and (2) providing insulation that minimizes head loss across the anterior abdominal wall. The greater omentum also contains numerous lymph nodes. These protect the body from foreign proteins, toxins, or pathogens that evade the defenses of the digestive tract. A thick mesentery, the *mesentery proper*, suspends all but the first 25 cm of the small intenstine. This provides stability while permitting a certain amount of independemnt movement. The *mesocolon* is a mesentery attached to the large intestine. The middle portion of the large intestine (the transverse colon) is suspended by the *transverse mesocolon*. The sigmoud colon, which leads to the rectum and anus, s suspended by the *sigmoid mesocolon*. During embryonic development, the dorsal mesentery of the ascending colon, descending colon, and rectum fuses to the posterior body wall. This fused mesentery fixes them in position. These organs are now secondarily retroperitoneal, and the visceral peritoneum covers only the anterior surfaces and portions of their lateral surfaces.